I. Field of the Invention
The present invention relates generally to cellular communications systems in which are disposed multiple base stations, each of which broadcasts a distinguishing pilot signal. More particularly, the present invention relates to a novel and improved technique of searching for and identifying the pilot signals of those base stations from which the signal strength received at a given location is sufficient to support communication.
II. Description of the Related Art
In conventional cellular telephone systems the available frequency band is divided into channels typically 30 KHz in bandwidth while analog FM modulation techniques are used. The system service area is divided geographically into cells of varying size. The available frequency channels are divided into sets with each set usually containing an equal number of channels. The frequency sets are assigned to cells in such a way as to minimize the possibility of co-channel interference. For example, consider a system in which there are seven frequency sets and the cells are equal size hexagons. A frequency set used in one cell will not be used in the six nearest or surrounding neighbors of that cell. Furthermore, the frequency set in one cell will not be used in the twelve next nearest neighbors of that cell.
In the conventional cellular telephone system, the handoff scheme implemented is intended to allow a call to continue when a mobile telephone crosses the boundary between two cells. The handoff from one cell to another is initiated when the receiver in the cell base station handling the call notices that the received signal strength from the mobile telephone falls below a predetermined threshold value. A low signal strength indication implies that the mobile telephone must be near the cell border. When the signal level fails below the predetermined threshold value, the base station asks the system controller to determine whether a neighboring base station receives the mobile telephone signal with better signal strength than the current base station.
In a code division multiple access (CDMA) cellular telephone system, a common frequency band is used for communication with all base stations in a system. The common frequency band allows simultaneously communication between a mobile station and more than one base station. Signals occupying the common frequency band are discriminated at the receiving station through the spread spectrum CDMA waveform properties based on the use of a high speed pseudonoise (PN) code. The high speed PN code is used to modulate signals transmitted from the base stations and the mobile stations. Transmitter stations using different PN codes or PN codes that are offset in time produce signals that can be separately received at the receiving station. The high speed PN modulation also allows the receiving station to receive a signal from a single transmitting station where the signal has traveled over several distinct propagation paths.
The common frequency band utilized throughout a CDMA cellular communication system allows the mobile station to remain in communication with more than one cellular base station during the handoff. In this environment communication between the mobile station and the other user is uninterrupted by the eventual handoff from the base station corresponding to the cell from which the mobile station is exiting, to the base station corresponding to cell into which the mobile station is entering. This type of handoff may be considered as a "soft" handoff in communications between cell base stations with the mobile wherein two or more base stations, or sectors of base stations, transmit concurrently to the mobile station. Similar are the techniques for a handoff between a sector of one cell and another cell, and a handoff between sectors of a same cell base station for a sectorized cell.
The cellular system controller typically begins the base station diversity or so-called "soft handoff" process. The cellular system controller begins by assigning a modem located in the new base station to the call. This modem is given the PN address associated with the call between the mobile station and the current base station modem. The new base station modem assigned to service the call searches for and finds the mobile station transmitted signal. The new base station modem also begins transmitting a forward link signal to the mobile station. The mobile station's searcher element searches for this forward link signal according to the signal information provided by the old base station. When the mobile station acquires the new base station modem transmitted signal, the mobile station may continue to communicate through the two base stations. Another base station could be added in the same manner as the first new base station above. In this case the mobile station may continue to communicate through three base stations. This process can continue until the mobile station is in communication with one base station for each demodulation element that the mobile station contains and beyond.
Since the mobile station is communicating with the user via at least one base station at all times throughout a soft handoff there is no interruption in communications between the mobile station and the user. A soft handoff in communications therefore provides significant benefits in its inherent "make before break" communication over conventional "break before make" techniques employed in other cellular communication systems.
In an exemplary CDMA cellular telephone system, such as is described in U.S. Pat. No. 5,267,261, entitled MOBILE STATION ASSISTED SOFT HANDOFF IN A CDMA CELLULAR COMMUNICATIONS SYSTEM, which is assigned to the assignee of the present invention and which is herein incorporated by reference, there is described a particular technique for effecting a soft handoff of the type described above. This technique is applicable to systems in which each base station transmits a spread spectrum "pilot" reference signal. These pilot signals are issued by the mobile stations to obtain initial system synchronization and to provide robust time, frequency, and phase tracking of the base station transmitted signals. The pilot signal transmitted by each base station in a system may use the same PN code but with a different code phase offset, meaning that the PN codes transmitted by neighboring base stations are identical but skewed in time with respect to one another. Phase offset allows the pilot signals to be distinguished from one another according to the base station from which they originate.
The system of the U.S. Pat. No. 5,267,261 contemplates maintaining within the mobile station several lists of base stations from which the received signal strength exceeds predetermined levels. The process of searching for base station pilot signals may be streamlined by defining four distinct sets of pilot offsets: the Active Set, the Candidate Set, the Neighbor Set and the Remaining Set. The Active Set identifies the base station(s) or sector(s) through which the mobile station is communicating. The Candidate Set identifies the base station(s) or sector(s) for which the pilots have been received at the mobile station with sufficient signal strength to make them members of the Active Set, but have not been placed in the Active Set by the base station(s). The Neighbor Set identifies the base station(s) or sector(s) which are likely candidates for the establishment of communication with the mobile station. The Remaining Set identifies the base station(s) or sector(s) having all other possible pilot offsets in the current system, excluding those pilot offsets currently in the Active, the Candidate and Neighbor sets.
After a call is initiated the mobile station continues to scan the pilot signals transmitted by base stations located in neighboring cells. Pilot signal scanning continues in order to determine if one or more of the neighboring base station transmitted pilot signals rises above a predetermined threshold, a level which is indicative that communications may be supported between the base station and the mobile station. When the pilot signal transmitted by a base station located in a neighboring cell rises above the threshold, it serves as an indication to the mobile station that a handoff should be initiated. In response to this pilot signal strength determination, the mobile station generates and transmits a control message to the base station presently servicing the call. This control message is relayed on to the system controller, which determines whether a handoff procedure should be initiated based on the availability of system resources.
In the above-mentioned CDMA system of U.S. Pat. No. 5,267,261 the process of placing a Neighbor Set base station member in the Candidate Set proceeds as follows. The pilot signal from the base station is first compared with a predefined threshold value. Upon the mobile station making a determination that the measured value exceeds a predefined threshold, the mobile station control processor generates and transmits a corresponding Pilot Strength Measurement Report Message. This Report Message is received by the base station with which the mobile station is currently in communication, and is forwarded to the system controller.
The decision for placing a Candidate Set member into the Active Set is made by the system controller. For example, when the measured Candidate pilot is of a signal strength which exceeds the signal strength of one other Active Set member pilot by a predetermined value it may join the Active Set. In an exemplary system there are limits placed on the number of Active Set members. Should the addition of a pilot to the Active Set exceed the Active Set limit, the weakest Active Set pilot may be removed to another set.
Unfortunately, however, conventional pilot strength measurement techniques tend to erroneously identify pilot signals of insufficient energy as exceeding the predefined Candidate Set signal strength threshold. Such erroneous pilot strength measurements may result in "false alarms", in which a member of the Neighbor Set is improperly added to the Candidate Set. This improper addition may in turn lead to the occurrence of a "false" handoff, i.e., a call transfer to a base station incapable of establishing communication with the mobile unit.
Accordingly, it is an object of the present invention to provide an improved method of searching for and identifying only those pilot signals transmitted by viable base station handoff candidates.